Could you post some 100% corner pics at say f5.6
I had a look in my LR library and I dont have anything shot at F5.6 - However, here is a 100% crop from the extreme bottom left hand corner. Shot at F14, ISO 100 on the 1DSMK3 - tripod mounted, mirror lock-up, live view for critical focus. I am really anal about focus, and usually spend over a minute tweaking at 10x magnification in live view.
The shot itself is of a salt flat, and salt crystals, so its a good candidate for fine detail.
There has been no sharpening applied to this crop - It was shot RAW, imported in LR, then exported as a TIFF to CS3, cropped and save to jpeg on maximum setting.